Novel Dimeric o-Carboranyl Triarylborane: Intriguing Ratiometric Color-Tunable Sensor via Aggregation-Induced Emission by Fluoride Anions

Abstract

A dimeric o-carboranyl triarylborane compound (2) with a biphenylene bridge group was prepared and characterized. Also, its solid-state structure was determined via X-ray diffraction. Treatment of 2 with an excess amount of KF in the presence of 18-crown-6 formed a dimer-type potassium salt, [2·F2]­[K·18-crown-6]2; its structure was fully confirmed by multinuclear NMR spectroscopy. UV–vis titration experiments carried out in THF showed that 2 binds fluoride ions with a binding constant (K) of 8.5 × 105 M–1. The linear decline of the UV/vis absorption of 2 upon titration with fluoride suggested that the triarylborane moieties acted as independent binding sites, which were not affected by each other. Contrary to a single emission (λem = 376 nm) of 2 assignable to an intramolecular charge transfer (ICT) transition at 298 K in THF, a broad low-energy emission band was additionally observed at 77 K, which is dominant in the film state. The TD-DFT calculation on the first excited singlet state (S1) of 2 shows that the low-energy emission band originates from the CT nature between carborane and triarylborane groups. Aggregation-induced emission (AIE) of 2 was clearly confirmed by enhanced photoluminescence intensity (λem = 489 nm) upon increasing the water fraction (f w) in the THF solution of 2, and it further accounts for the intense emission in the solid state. Interestingly, the emission spectrum of a film sample of 2 upon addition of two equivalents of fluoride ion was was mostly similar to that of [2·F2]­[K·18-crown-6]2, indicating that the ICT-based AIE nature of 2 could be red-shifted by fluoride binding.

Solid‐State Thermochromic Luminescence through Twisted Intramolecular Charge Transfer and Excimer Formation of a Carborane−Pyrene Dyad with an Ethynyl Spacer

“…Boron cluster compounds have recently attracted significant attention as a key component in the development of stimuli‐responsive luminescent solid films, which hold great promise as versatile sensors and probes . To obtain clear responses, such materials need to show dramatic luminochromism that is triggered by the target stimuli.…”

Recent Progress in the Development of Solid‐State Luminescent o‐Carboranes with Stimuli Responsivity

“…Other types of AIE‐active o ‐carborane derivatives are listed in Table and Scheme . Influences on AIE properties were investigated by changing the substituents on the o ‐carborane unit ( 44 – 75 ) . For these compounds, solid‐state excimer emission, single‐molecule white emission, aggregation‐induced electrochemiluminescence, and thermally activated delayed fluorescence were observed.…”

Enhancement of Aggregation-Induced Emission by Introducing Multiple o-Carborane Substitutions into Triphenylamine

“…o -Carborane [ 5 , 6 , 7 , 8 , 9 , 10 ] is an icosahedral cluster composed of 2 carbon and 10 boron atoms, and it has recently attracted attention as a key component for constructing solid-state luminescent materials [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Because of the electron-deficient nature of the boron cluster, intense emission from the intramolecular charge transfer (ICT) state can be observed from the donor–acceptor system with electron-donating units such as triphenylamine and its structural analogues [ 23 , 24 , 25 , 26 ].…”

“…Because of the electron-deficient nature of the boron cluster, intense emission from the intramolecular charge transfer (ICT) state can be observed from the donor–acceptor system with electron-donating units such as triphenylamine and its structural analogues [ 23 , 24 , 25 , 26 ]. In particular, owing to steric hindrance of the sphere shape of the cluster, suppression of ACQ was often induced in the condensed state, followed by intense solid-state emission [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 27 , 28 , 29 ]. Indeed, applicability of the modified o -carboranes for organic light-emitting devices has been demonstrated from the recent studies [ 30 , 31 ].…”